As is well known, the catalytic conversion of virgin gas oils containing aromatic, naphthenic and paraffinic molecules results in the formation of a variety of distillates that have ever-increasing utility and importance in the petrochemical industry. The economic and utilitarian value, however, of the residual fraction of the cat cracking processes has not increased to the same extent as the light overhead fractions has. One potential use for such cat cracker bottoms is in the manufacture of carbon artifacts. As is well known, carbon artifacts have been made by pyrolyzing a wide variety of organic materials. Indeed, one carbon artifact of particularly important commercial interest today is carbon fiber. Hence, particular reference is made herein to carbon fiber technology. Nevertheless, it should be appreciated that this invention has applicability to carbon artifact formation generally and, most particularly, to the production of shaped carbon articles in the form of filaments, yarns, films, ribbons, sheets, and the like.
Referring now in particular to carbon fibers, suffice it to say that the use of carbon fibers in reinforcing plastic and metal matrices has gained considerable commercial acceptance where the exceptional properties of the reinforcing composite materials, such as their higher strength to weight ratio, clearly offset the generally higher costs associated with preparing them. It is generally accepted that large scale use of carbon fibers as a reinforcing material would gain even greater acceptance in the marketplace if the costs associated with the formation of the fibers could be substantially reduced. Thus, the formation of carbon fibers from relatively inexpensive carbonaceous pitches has received considerable attention in recent years.
Many carbonaceous pitches are known to be converted at the early stages of carbonization to a structurally ordered optically anisotropic spherical liquid crystal called mesophase. The presence of this ordered structure prior to carbonization is considered to be a significant determinant of the fundamental properties of any carbon artifact made from such a carbonaceous pitch. Indeed, the ability to generate high optical anisotropicity during processing is accepted, particularly in carbon fiber production, as a prerequisite to the formation of high quality products. Thus, one of the first requirements of a feedstock material suitable for carbon artifact manufacture, and particularly carbon fiber production, is its ability to be converted to a highly optically anisotropic material.
In addition to being able to develop a highly ordered structure, suitable feedstocks for carbon artifact manufacture, and in particular carbon fiber manufacture, should have relatively low softening points rendering them suitable for being deformed and shaped into desired articles. Thus, in carbon fiber manufacture, a suitable pitch which is capable of generating the requisite highly ordered structure also must exhibit sufficient viscosity for spinning. Unfortunately, many carbonaceous pitches have relatively high softening points. Indeed, incipient coking frequently occurs in such materials at temperatures where they have sufficient viscosity for spinning. The presence of coke, however, or other infusible materials and/or undesirably high softening point components generated prior to or at the spinning temperatures are detrimental to processability and are believed to be detrimental to product quality. Thus, for example, U.S. Pat. No. 3,919,376 discloses the difficulty in deforming pitches which undergo coking and/or polymerization at the softening temperature of the pitch.
Another important characteristic of the feedstock for carbon artifact manufacture is its rate of conversion to a suitable optically anisotropic material. For example, in the above-mentioned U.S. patent, it is disclosed that 350.degree. C. is the minimum temperature generally required to produce mesophase from a carbonaceous pitch. More importantly, however, is the fact that at least one week of heating is necessary to produce a mesophase content of about 40% at that minimum temperature. Mesophase, of course, can be generated in shorter times by heating at higher temperatures. However, as indicated above, at temperatures in excess of about 425.degree. C., incipient coking and other undesirable side reactions do take place which can be detrimental to the ultimate product quality.
In U.S. Pat. No. 4,208,267, it has been disclosed that typical graphitizable carbonaceous pitches contain a separable fraction which possesses very important physical and chemical properties insofar as carbon fiber processing is concerned. Indeed, the separable fraction of typical graphitizable carbonaceous pitches exhibits a softening range and viscosity suitable for spinning and has the ability to be converted rapidly at temperatures in the range generally of about 230.degree. C. to about 400.degree. C. to an optically anisotropic deformable pitch containing greater than 75% of a liquid crystalline type structure. Unfortunately, the amount of separable fraction present in well known commercially available petroleum pitches, such as Ashland 240 and Ashland 260, to mention a few, is exceedingly low. For example, with Ashland 240, no more than about 10% of the pitch constitutes a separable fraction capable of being thermally converted to a deformable anisotropic phase.
In U.S. Pat. No. 4,184,942, it has been disclosed that the amount of that fraction of typical graphitizable carbonaceous pitches that exhibits a softening point and viscosity which is suitable for spinning and which has the ability to be rapidly converted at low temperatures to highly optically anisotropic deformable pitch can be increased by heat soaking the pitch, for example at temperatures in the range of 350.degree. C. to 450.degree. C., until spherules visible under polarized light begin to appear in the pitch. The heat soaking of such pitch results in an increase in the amount of the fraction of the pitch capable of being converted to an optically anisotropic phase.
In U.S. Pat. No. 4,219,404, it has been disclosed that the polycondensed aromatic oils present in isotropic graphitizable pitches are generally detrimental to the rate of formation of highly optically anisotropic material in such feedstocks when they are heated at elevated temperatures and that, in preparing a feedstock for carbon artifact manufacture, it is particularly advantageous to remove at least a portion of the polycondensed aromatic oils normally present in the pitch simultaneously with, or prior to, heat soaking of the pitch for converting it into a feedstock suitable in carbon artifact manufacture.
More recently, in copending application Ser. No. 143,136, filed Apr. 23, 1980 now U.S. Pat. No. 4,271,006, a process has been disclosed for converting cat cracker bottoms to a feed stock suitable in carbon artifact manufacture. Basically, the process requires stripping cat cracker bottoms of fractions boiling below 400.degree. C. and thereafter heat soaking the residue followed by vacuum stripping to provide a carbonaceous pitch.